The present invention relates to a fire alarm system, and more particularly to a fire alarm system in which a plurality of fire detectors are connected to each of the alarm lines which are extended from a central monitoring station to monitoring sections, said fire detectors short-circuiting the alarm lines with low impedance when any fire detector detects a fire, and the signal from any of the fire detectors is optically displayed by means of light emitting diodes or the like.
In conventional systems of this type, an accidental disconnection or a failure in the wiring is detected by monitoring the level of the current which flows through a resistor which is connected to the distal end of each alarm line. This system has a defect in that extra electric power is consumed for such failure detecting operation. Recently, many high and very large buildings, and other structures are being built, so that extensive sections have to be monitored and the number of required alarm lines inevitably has to be increased. Under the circumstances, the above-mentioned defect of conventional fire alarm systems is no longer negligible.
To diminish the above defect, there is known a method wherein a diode is connected in series with the above-stated resistor so that normally no current flows, and a voltage of the reverse polarity is applied to the alarm lines only when the lines are to be tested for disconnection or breakage. However, even this method has defects in that it is impossible to constantly monitor the wiring for breakage and disconnection, and that fire detectors of some types may be broken by the voltage of reverse polarity.
It is, therefore, the object of the present invention to provide a fire alarm system in which disconnection in the wiring can be monitored all the time with less consumption of electric power.